AC; for it is contained by DA, AC, of which AD is equal to Book II. AB; and CE is contained by AB, BC, for BE is equal to AB; therefore the rectangle contained by AB, AC, together with the rectangle AB, BC, is equal to the square of AB. If therefore a straight line, &c. Q. E. D. PROP. III. THEOR. IF a straight line be divided into any two parts, the rectangle contained by the whole and one of the parts is equal to the rectangle contained by the two parts, together with the square of the foresaid part. Let the straight line AB be divided into any two parts in the point C; the rectangle AB, BC is equal to the rectangle AC, CB, together with the square of BC. A C Ba 46. 1. Upon BC describe a the square CDEB, and produce ED to F, and through A drawb AF parallel to CD or BE; then the rectangle AE is equal to the rectangles AD, СЕ: and AE is the rectangle contained by AB, BC, for it is contained by AB, BE, of which BE is equal to BC; and AD is contained by AC, CB, for CD is equal to CB; and DB F is the square of BC; therefore the rectangle AB, BC is equal to the rectangle AC, CB, together with the square of BC. If therefore a straight line, &c. Q. E. D. PROP. IV. THEOR. IF a straight line be divided into any two parts, the square of the whole line is equal to the squares of the two parts, together with twice the rectangle contained by the parts. Let the straight line AB be divided into any two parts in C; the square of AB is equal to the squares of AC, CB, and to twice the rectangle contained by AC, CB, b 31. 1. Book II. a 46. 1. b 31. 1. с 29. 1. d 5. 1. Upon AB describe a the square ADEB, and join BD, and ☑ through C draw b CGF parallel to AD or BE, and through G draw HK parallel to AB or DE: and because CF is parallel to AD, and BD falls upon them, the exterior angle BGC is equal c to the interior and opposite angle ADB; but ADB is equal d to the angle ABD, because BA is equal to AD, being sides of a square; wherefore the angle CGB C B A is equal to the angle GBC; and there e 6. 1. fore the side BC is equale to the side f34.1. CG: but CB is equal f also to GK, G and CG to BK; wherefore the figure H K D wherefore GCB is a right angle; and therefore also the angles f CGK, GKB, opposite to these, are right angles, and CGKB is rectangular: but it is also equilateral, as was demonstrated; wherefore it is a square, and it is upon the side CB: for the same reason HF also is a square, and it is upon the side HG, which is equal to AC: therefore HF, CK are the squares of g 43. 1. AC, CB; and because the complement AG is equals to the complement GE, and that AG is the rectangle contained by AC, CB, for GC is equal to CB; therefore GE is also equal to the rectangle AC, CB: wherefore AG, GE are equal to twice the rectangle AC, CB: and HF, CK are the squares of AC, CB; wherefore the four figures HF, CK, AG, GE are equal to the squares of AC, CB, and to twice the rectangle AC, CB; but HF, CK, AG, GE make up the whole figure ADEB, which is the square of AB: therefore the square of AB is equal to the squares of AC, CB, and twice the rectangle AC, CB. Wherefore, if a straight line, &c. Q. E. D. COR. From the demonstration, it is manifest, that the parallelograms about the diameter of a square are likewise squares. Book II. PROP. V. THEOR. IF a straight line be divided into two equal parts, and also into two unequal parts; the rectangle contained by the unequal parts, together with the square of the line between the points of section, is equal to the square of half the line. Let the straight line AB be divided into two equal parts in the point C, and into two unequal parts at the point D; the rectangle AD, DB, together with the square of CD, is equal to the square of CB. Upon CB describe a the square CEFB, join BE, and through a 46. 1. D draw b DHG parallel to CE or BF; and through H draw b 31. 1. KLM parallel to CB or EF; and also through A draw AK parallel to CL or BM: and because the complement CH is equal to the complement HF, to each of these add DM; c 43. 1. therefore the whole CM rectangle contained by AD, 1 DB, for DH is equale to DB; and DF together with CH is the e Cor. 4.2. gnomon CMG; therefore the gnomon CMG is equal to the rectangle AD, DB: to each of these add LG, which is equal e to the square of CD; therefore the gnomon CMG, together with LG, is equal to the rectangle AD, DB, together with the square of CD: but the gnomon CMG and LG make up the whole figure CEFB, which is the square of CB: therefore the rectangle AD, DB, together with the square of CD, is equal to the square of CB. Wherefore, if a straight line, &c. Q. E. D. From this proposition it is manifest, that the difference of the squares of two unequal lines AC, CD, is equal to the rectangle contained by their sum and difference. Book II. PROP. VI. THEOR. IF a straight line be bisected, and produced to any point, the rectangle contained by the whole line thus produced, and the part of it produced, together with the square of half the line bisected, is equal to the square of the straight line which is made up of the half and the part produced. Let the straight line AB be bisected in C, and produced to the point D; the rectangle AD, DB, together with the square of CB, is equal to the square of CD. a 46. 1. Upon CD describe a the square CEFD, join DE, and through b 31. 1. B draw & BHG parallel to CE or DF, and through H draw KLM parallel to AD or EF, and also through A draw AK parallel to CL or DM: and because AC is A C B D angle contained by AD, DB, E GF e Cor. 4.2. for DM is equale to DB: therefore the gnomon CMG is equal to the rectangle AD, DB: add to each of these LG, which is equal to the square of CB; therefore the rectangle AD, DB, together with the square of CB, is equal to the gnomon CMG and the figure LG: but the gnomon CMG and LG make up the whole figure CEFD, which is the square of CD; therefore the rectangle AD, DB, together with the square of CB, is equal to the square of CD. Wherefore, if a straight line, &c. Q. E. D. PROP. VII. THEOR. IF a straight line be divided into any two parts, the squares of the whole line, and of one of the parts, are equal to twice the rectangle contained by the whole and that part, together with the square of the other part. Let the straight line AB be divided into any two parts in the point C; the squares of AB, BC are equal to twice the rect- Book II. angle AB, BC, together with the square of AC. Upon AB describe a the square ADEB, and construct the a 46. 1. figure as in the preceding propositions: and because AG is equal to GE, add to each of them CK; the whole AK is b 43. 1. therefore equal to the whole CE; therefore AК, СЕ are double of A C B AK: but AK, CE are the gnomon therefore the gnomon AKF, toge- G K c eor. 4.2. D FE PROP. VIII. THEOR. IF a straight line be divided into any two parts, four times the rectangle contained by the whole line, and one of the parts, together with the square of the other part, is equal to the square of the straight line which is made up of the whole and that part. Let the straight line AB be divided into any two parts in the point C; four times the rectangle AB, BC, together with the square of AC, is equal to the square of the straight line made up of AB and BC together. Produce AB to D, so that BD be equal to CB, and upon AD describe the square AEFD; and construct two figures such as in the preceding. Because CB is equal to BD, and that CB is equal to GK, and BD to KN; therefore GK is a 34. 1. H |